There are many instances in which enhanced braking capabilities from material handling equipment are desired. Such capabilities are particularly desirable for crawler-type drive tractors with right-hand and left-hand crawler drive tracks. Because such crawler-type drive tractors are used for a variety of material handling functions in areas having restricted space and/or irregular or hilly terrain, effective braking and precise control for manueverabilty are required for efficient operation.
Transmission systems for crawler-type drive tractors known previously have typically included a clutch and a steering brake for each of the crawler tracks. These clutches and steering brakes are selectively and hydraulically actuated through operator-controlled mechanical linkages. The mechanical linkages operate the necessary hydraulic cylinders and valves which effect operation of the track clutches and brakes.
In many crawler-track transmission systems currently in use, hydraulic actuation of the brake for one of the crawler tracks results in disengagement of the clutch for that track. As a result, the track is disconnected from the internal combustion engine of the implement. Clutch disengagement is usually effected hydraulically by providing hydraulic fluid lines which are operatively associated with the brake cylinder for one of the crawler tracks and its respective clutch. Disengagement of the clutch for the track being braked, while the clutch of the other crawler track remains engaged and that track driven, permits the operator of the implement to execute a so-called pivot turn for manuevering the implement.
While many crawler track transmission system operate as described above, it will be appreciated that in some instances, disengagement of the crawler track clutches in connection with application of the track brakes is not desirable. This is particularly true when the crawler implement is being used on hilly or irregular terrain. When used on such terrain, it is frequently desirable to take advantage of the braking effect provided by the implement's internal combustion engine to brake the implement. Of course, this requires that the clutches operatively connecting the crawler tracks with engine remain engaged.
In one known crawler track transmission system, as disclosed in commonly-assigned U.S. Pat. No. 4,479,563, pressure sensors are provided for each of the crawler track brake lines. Each of the pressure sensors generates an electrical signal indicating when the associated brake pedal has been depressed. The electrical signal from each pressure sensor is coupled to an electrically actuated lock valve associated with the clutch for the other crawler track. Actuation of both right-hand and left-hand track brakes simultaneously prevents the normal disengagement of the track clutches. This then permits the engine of the tractor to assist in braking. This known system is a combined electrical-hydraulic system.
While the above-noted, known crawler track transmission system provides the desired function of coupling the crawler tracks with the engine during the time when both brake pedals are simultaneously depressed, it is desirable to provide a control system which is non-electric and which provides the same function. It is also desirable that any such non-electric system not impair normal manueverability of the implement. Depressing only one of the implement brake pedals should still result in normal disengagement of the respective clutch for the track being braked. This will permit the operator of the implement to continue to efficiently execute the pivot turns which are necessary for manuevering the vehicle. In addition, any such hydraulic system should be readily fabricated and installed, and should be straightforward in construction to facilitate reliable operation under the severe operating condition to which material handling implements are typically subjected.